1. Field of the Invention
The present invention relates to a mold clamping system for an injection molding machine and, more particularly to apparatus to support a short-stroke hydraulic ram in a two-platen injection molding machine.
2. Description of the Related Art
The clamping system of a typical injection molding machine generally comprises two rectangular stationary platens; a front platen adjacent the machine's injection unit, and a rear platen, with tie rods connecting the four corners of the two platens. A movable platen is located between the stationary platens and is supported by the tie rods in a way that allows for translational movement between the two stationary platens. A mold constructed in two "halves" is mounted so that one half is attached to the front stationary platen and the other half attached to the movable platen. The force for the clamping system is usually provided by a hydraulic cylinder that acts directly or via a toggle mechanism to open and close the mold halves by applying the appropriate force to the movable platen. With the mold held closed by the clamping system, plastic melt is injected into the cavity formed by the mold halves, forming a molded product.
One of the drawbacks of the conventional, three platen injection molding machines is that the overall length of the machine is relatively long, meaning that the machine occupies a significant amount of valuable space on the manufacturing floor.
Accordingly, the prior art has proposed various "two-platen" constructions for injection molding machines that have only one stationary platen. More specifically, prior art two platen clamping systems generally include a front (stationary) platen, a movable platen, four tie rods fixedly connected to one of the platens, releasable nuts attached to the other platen to engage the tie rods, means for traversing the movable platen, and one or more clamping cylinders to hold the mold closed. In operation, the movable platen is traversed into position to close the mold; the nuts are then closed on the tie rods to provide a rigid link between the two platens; and clamp tonnage is applied by the clamp cylinder in preparation for injection of the plastic melt.
While two-platen machines have the advantage of significantly shorter overall length, their configuration also presents potential problems. For example, since the two-platen design is primarily used for larger machines, the associated molds tend to also be very large and heavy. The weight of the mold must be supported by the platens in such a way that the platen surfaces remain parallel when the mold is open, so that it will close smoothly and properly form the mold cavity. However, the eccentric weight makes this difficult, particularly since the tie rods are fixedly supported at only one end when the movable platen is in motion.
In addition, most prior art two-platen machines have employed clamping cylinders associated with all four tie rods, thereby applying force to the mold via the tie rods. Clamping the mold in this manner tends to be rather complex since it requires that four clamping mechanisms be carefully synchronized to assure that uniform pressure is applied to the mold.
A few prior art attempts have been made to apply the clamping force centrally through a mold mounting surface. However, these clamping systems have not been satisfactory because of their failure to maintain alignment of the mold surfaces. In particular, due to the variation in weights of different molds, combined with the manufacturing tolerances of the components and associated running clearances connected with assembly, the mold mounting surface tilts from vertical after the mold is attached. In other words, the prior art has failed to provide a means of compensating for the manufacturing tolerance stack-up.